This invention relates to semiconductor devices and more specifically relates to a termination structure for a MOSgated device.
MOSgated semiconductor devices such as MOSFETS, IGBTs and the like must have termination structures surrounding the active device area to prevent breakdown at the peripheral edge of the die or chip. Thus, the breakdown voltage of MOSgated device is frequently limited by the termination rather than by the active area. Termination structures employ conductive field plates and floating guard ring diffusions which are laterally separated from the guard rings. If the termination has a sufficiently high breakdown voltage, it becomes possible to increase the concentration of the epitaxially formed drift region of the active area, thus reducing its resistance RDSON when the device is turned on.
Such terminations use a certain silicon area, thus reducing the percentage of the chip area devoted to active area. Thus, as terminations are made to support higher breakdown voltage, they also require more area, thus reducing the active area of a given chip and offsetting the reduced RDSON benefit.
It would be desirable to provide a termination for a MOSgated device which employs both field plates and guard ring diffusions while taking a reduced area of a given chip or die.
In accordance with the invention the guard ring diffusions which are usually spaced from the field plate, and which provide the benefit of grading the electric field from the border of the active region to the street or edge of the die are disposed beneath the field plate. It has been found that the new termination permits a 25% reduction in RDSON of a MOSFET of a given voltage rating and die size because it uses less area, permitting an increase in active area for a given die size, and because it enables an increase in the concentration of the body or drift region.
In particular, for a power MOSFET rated at a breakdown voltage of 200 volts, and a rectangular die area of 2.8 mm by 3.6 mm the novel termination permitted a decrease in termination area from 1.7 mm2 to 1.1 mm2 and a decrease in the resistivity of the epitaxial silicon body region of 5.7 ohm cm to 4.7 ohm cm. The on resistance of the die was reduced from about 0.19 ohms to 0.14 ohms.